This invention relates to use of adsorbents in purification of relatively impure olefins such as are typically produced by thermal cracking of suitable hydrocarbon feedstocks. More particularly, this invention concerns purification by passing an olefinic stream, containing alkanes, small amounts of acetylenic impurities, carbon oxides and/or other organic components, which are typically impurities in cracked gas oil, in contact with an adsorbent comprising a crystalline titanium silicate under conditions suitable for adsorption of olefins and/or alkynes.
Generally, this invention is directed to separating useful alkenes (olefins) and/or alkynes from alkanes (paraffins) of the same carbon content and is more specifically directed to separating ethylene or propylene from mixed streams of ethane/ethylene or propane/propylene, respectively, using CTS titanium silicate adsorbents.
As is well-known, olefins, or alkenes, are a homologous series of hydrocarbon compounds characterized by having a double bond of four shared electrons between two carbon atoms. The simplest member of the series, ethylene, is the largest volume organic chemical produced today. Importantly, olefins including ethylene, propylene and smaller amounts of butadiene, are converted to a multitude of intermediate and end products on a large scale, mainly polymeric materials.
Commercial production of olefins is almost exclusively accomplished by pyrolysis of hydrocarbons in tubular reactor coils installed in externally fired heaters. Thermal cracking feedstocks include streams of ethane, propane or a hydrocarbon liquid ranging in boiling point from light straight-run gasoline through gas oil. Because of the very high temperatures employed, commercial olefin processes invariably coproduce significant amounts of acetylene. Required separation of the acetylene from the primary olefin can considerably increase the plant cost.
In a typical ethylene plant, the cracking represents about 25% of the cost of the unit, while the compression, heating, dehydration, recovery and refrigeration sections represent the remaining percentage of the total. This endothermic process is carried out in large pyrolysis furnaces with the expenditure of large quantities of heat, which is provided in part by burning the methane produced in the cracking process. After cracking, the reactor effluent is put through a series of separation steps involving cryogenic separation of products such as ethylene and propylene. The total energy requirements for the process are thus very large, and ways to reduce it are of substantial commercial interest. In addition, it is of significant interest to reduce the amounts of methane and heavy fuel oils produced in the cracking processor and utilize them for other than for their fuel value.
Hydrocarbon cracking is carried out using a feed, which is ethane, propane, or a hydrocarbon liquid ranging in boiling point from light straight-run gasoline through gas oil. Ethane, propane, liquid naphthas, or mixtures thereof are preferred feed to a hydrocarbon cracking unit. Hydrocarbon cracking is generally carried out thermally in the presence of a dilution steam in large cracking furnaces which are heated, at least in part, by burning methane and other waste gases from the olefins process resulting in large amounts of NO, pollutants. The hydrocarbon cracking process is very endothermic and requires large quantities of heat per pound of product. However, newer methods of processing hydrocarbons utilize, at least to some extent, catalytic processes, which are better able to be tuned to produce a particular product slate. The amount of steam used per pound of feed in the thermal process depends to some extent on the feed used and the product slate desired. Typically, steam pressures are in the range of about 30 lbs. per sq. in. to about 80 lbs. per sq. in. (psi), and amounts of steam used are in the range of about 0.2 lbs. of steam per pound of feed to 0.7 lbs. of steam per pound of feed. The temperature, pressure, and space velocity ranges used in thermal hydrocarbon cracking processes depend to some extent upon the feed used and the product slate desired, which are well-known and may be appreciated by one skilled in the art. The type of furnace used in the thermal cracking process is also well-known.
Several methods are known for separation of an organic gas containing unsaturated linkages from gaseous mixtures. These include, for instance, cryogenic distillation, liquid absorption, membrane separation and the so-called xe2x80x9cpressure swing adsorptionxe2x80x9d in which adsorption occurs at a higher pressure than the pressure at which the adsorbent is regenerated. Cryogenic distillation and liquid absorption are common techniques for separation of carbon monoxide and alkenes from gaseous mixtures containing molecules of similar size, e.g. nitrogen or methane. However, both techniques have disadvantages such as high capital cost and high operating expenses. For example, liquid absorption techniques suffer from solvent loss and need a complex solvent make-up and recovery system.
Olefin-paraffin separations represent a class of most important and also most costly separations in the chemical and petrochemical industry. Cryogenic distillation has been used for over 60 years for these separations. They remain to be the most energy-intensive distillations because of the close relative volatilities. For example, ethane-ethylene separation (c2 splitter) is carried out at about xe2x88x9225xc2x0 C. and 320 lbs. per sq. in. gage pressure (psig) in a column containing over 100 trays, and propane-propylene separation is performed by an equally energy-intensive distillation at about xe2x88x9230xc2x0 C. and 30 psig. The energy costs in olefin/paraffin separations are enormous. Recent revamps of ethylene plants have involved replacing distillation trays in the towers and heat exchange tubing in condensers and reboilers to reduce energy costs. New methods of process control and manipulation of feed point, product draw, de-ethanizer processing have all been used to control energy usages in an ethylene plant. Obviously, new methods of olefin/paraffin separation, which are less energy intensive as the present distillations, would be welcomed and could replace or at least augment the present C2 splitter distillation processes.
Listed below are the mole weight and atmosphere boiling points for the light products from thermal cracking and some common compounds potentially found in an olefins unit. Included are some compounds, which have similar boiling temperatures to cracked products and may be present in feedstocks or produced in trace amounts during thermal cracking.
Recently, the trend in the hydrocarbon processing industry is to reduce commercially acceptable levels of impurities in major olefin product streams, i.e., ethylene, propylene, and hydrogen. Need for purity improvements are directly related to increasing use of higher activity catalysts for production of polyethylene and polyproypropylene, and, to a limited, extent other olefin derivatives.
It is known that acetylene can be selectively hydrogenated and thereby removed from such product streams by passing the product stream over an acetylene hydrogenation catalyst in the presence of molecular hydrogen, H2. However, these hydrogenation processes typically result in the deposition of carbonaceous residues or xe2x80x9cgreen oilxe2x80x9d on the catalyst, which deactivates the catalyst. Therefore, acetylene hydrogenation processes for treating liquid or liquefiable olefins and diolefins typically include an oxygenation step or a xe2x80x9cburnxe2x80x9d step to remove the deactivating carbonaceous residues from the catalyst, followed by a hydrogen reduction step to reactivate the hydrogenation catalyst. For example, see U.S. Pat. No. 3,755,488 to Johnson, et. al.; U.S. Pat. No. 3,792,981 to Hettick, et. al.; U.S. Pat. No. 3,812,057 to Morgan; and U.S. Pat. No. 4,425,255 to Toyoda. However, U.S. Pat. Nos. 3,912,789 and 5,332,705 state that by using selected hydrogenation catalysts containing palladium, at least partial regeneration can be accomplished using a hydrogenation step alone at high temperatures (600xc2x0 to 700xc2x0 F.) and in the absence of an oxygenation step.
Selective hydrogenation of the about 2,000 to 4,000 parts per million of acetylenic impurities to ethylene is generally a crucial operation for purification of olefins produced by thermal steam cracking. Typical of a small class of commercially useful catalysts are materials containing very low levels of an active metal supported on an inert carrier, for example, a particulate bed having less than about 0.03% (300 ppm) palladium supported on the surface skin of carrier pellets having surface area of less than about 10 m2/gm.
Many commercial olefin plants using steam crackers use front-end acetylene converters, i.e. the hydrogenation unit is fed C3 and lighter cracked gas, which feed has a high enough concentration of hydrogen to easily hydrogenate the acetylenic impurities; however, when run improperly, will also hydrogenate a large fraction of the ethylene and propylene product. Both hydrogenation of acetylene and ethylene are highly exothermic.
Accelerated catalyst deactivation and thermal runaways caused by loss in catalyst selectivity are common problems, which plague acetylene converters. Such problems result in unscheduled shutdowns and increased costs to replace deactivated catalyst.
The problem of over-hydrogenation is aggravated because the rate constant for ethylene hydrogenation to ethane is 100 times faster than for the hydrogenation of acetylene to ethylene. As a means to avoid a C2H4 hydrogenation thermal runaway, acetylene, carbon monoxide and diolefins concentrations must be high enough to cover most active sites so none are left to adsorb ethylene.
In certain instances, it may be useful to recover acetylene from the thermally cracked hydrocarbon stream since acetylene is a valuable raw material. Unfortunately, the boiling point of acetylene is close to the other C2 hydrocarbons, ethane and ethylene, such that distillation is impractical. Liquid absorption of acetylene from a crude C2-stream is disclosed in U.S. Pat. No. 4,655,798. In U.S. Pat. No. 6,124,517, acetylene impurities are adsorbed from an olefin stream by passing the feed stream such as obtained from thermal cracking through a particulate bed of adsorbent comprising a support material having high surface area on which is dispersed at least one metallic element in the zero valent state such as copper or silver. A high surface area gamma-aluminum silica, active carbon, clay and zeolites are disclosed as the support material. The adsorbent as disclosed in the patent is for removing acetylenic impurities from ethylene or propylene streams and is not described as useful for separating the desired olefins from the homogolous paraffins or in other words, separating ethylene from ethane and replacing the conventionally used C2 splitter distillation process.
In commonly assigned U.S. Pat. No. 4,938,939, issued Jul. 3, 1990, Kuznicki disclosed a new family of synthetic, stable crystalline titanium silicate molecular sieve zeolites, which have a pore size of approximately 3-4 Angstrom units and a titania/silica mole ratio in the range of from 1.0 to 10. The entire content of U.S. Pat. No. 4,938,939 is herein incorporated by reference. Members of the family of titanium silicate molecular sieves, designated ETS-4, in the rare earth-exchanged form have a high degree of thermal stability of at least 450xc2x0 C. or higher depending on cationic form. ETS zeolites are highly adsorptive toward molecules up to approximately 3-5 Angstroms in critical diameter, e.g. water, ammonia, hydrogen sulfide, SO2, and n-hexane and are essentially non-adsorptive toward molecules, which are larger than 5 Angstroms in critical diameter.
The new family of microporous titanium silicates developed by the present assignee, and generically denoted as ETS, are constructed from fundamentally different building units than classical aluminosilicate zeolites. Instead of interlocked tetrahedral metal oxide units as in classical zeolites, the ETS materials are composed of interlocked octahedral chains and classical tetrahedral rings. In general, the chains consist of six oxygen-coordinated titanium octahedra and wherein the chains are connected three dimensionally via tetrahedral silicon oxide units or bridging titanosilicate units. The inherently different crystalline titanium silicate structures of these ETS materials have been shown to produce unusual and unexpected results when compared with the performance of aluminosilicate zeolite molecular sieves. For example, the counter-balancing cations of the crystalline titanium silicates are associated with the charged titania chains and not the uncharged rings, which form the bulk of the structure.
As synthesized, ETS-4 has an approximately 4 xc3x85 effective pore diameter. Reference to pore size or xe2x80x9ceffective pore diameterxe2x80x9d defines the effective diameter of the largest gas molecules significantly adsorbed by the crystal. This may be significantly different from, but systematically related to, the crystallographic framework pore diameter. For ETS-4, the effective pore is defined by eight-membered rings formed from TiO62xe2x88x92 octahedra and SiO4 tetrahedra. This pore is analogous to the functional pore defined by the eight-membered tetrahedral metal oxide rings in traditional small-pored zeolite molecular sieves.
The pores of ETS-4 formed by the eight-membered polyhedral TiO6 and SiO4 units are non-faulted in a singular direction, the b-direction, of the ETS crystal and, thus, fully penetrate the crystal, rendering the ETS-4 useful for molecular separations. Recently, however, researchers of the present assignee have discovered a new phenomenon with respect to ETS-4. In appropriate cation forms, the pores of ETS-4 can be made to systematically shrink from slightly larger than 4 xc3x85 to less than 3 xc3x85 during calcinations, while maintaining substantial sample crystallinity. These pores may be xe2x80x9cfrozenxe2x80x9d at any intermediate size by ceasing thermal treatment at the appropriate point and returning to ambient temperature. These materials having controlled pore sizes are referred to as CTS-1 (contracted titanosilicate-1) and are described in commonly assigned U.S. Pat. No. 6,068,682, issued May 30, 2000 herein incorporated by reference in its entirety. Thus, ETS-4 may be systematically contracted under appropriate conditions to CTS-1 with a highly controllable pore size in the range of 3-4 xc3x85. With this extreme control, molecules in this range may be separated by size, even if the sizes of the respective molecules are nearly identical. This profound change in adsorptive behavior is accompanied by systematic structural changes as evidenced by X-ray diffraction patterns and infrared spectroscopy. The systematic contraction of ETS-4 to CTS-1 to a highly controllable pore size has been named the Molecular Gate(trademark) effect. This effect is leading to the development of separation of molecules differing in size by as little as 0.1 Angstrom, such as N2/O2 (3.6 and 3.5 Angstroms, respectively), CH4/N2 (3.8 and 3.6 Angstroms), or CO/H2 (3.6 and 2.9 Angstroms). High pressure N2/CH4 separation systems are now being developed. In this latter system, pressure swing adsorption (PSA) is utilized to adsorb the nitrogen from the natural gas stream, and desorb the nitrogen from the titanium silicate molecular sieve. Besides the use of CTS-1 for size controlled adsorption, it is known that barium-exchanged ETS-4 has the ability to size discriminate molecules from each other. For example, U.S. Pat. No. 5,989,316, discloses the use of Ba-exchanged ETS-4 to separate nitrogen from methane. The entire content of U.S. Pat. No. 5,989,316 is incorporated herein by reference.
As disclosed in U.S. Pat. No. 6,068,682 the CTS-1 zeolites may be generated using increasing thermal treatments which systematically size-exclude ethane (about 3.6 to above 4 xc3x85), methane (about 3.8 xc3x85), argon (about 3.7. xc3x85), N2 (about 3.6 xc3x85), O2 (about 3.5 xc3x85), carbon dioxide (about 3.3 xc3x85) and water (about 2.7 xc3x85). Except for argon these sizes are Lennard-Jones kinetic diameters from Zeolite Molecular Sieves, Donald W. Breck Publishng Company, Malabar, Fla., 1984, p. 636. This reference lists argon as 3.4 xc3x85, but our size exclusion data repeatedly shows it behaves in a sieving system as being between 3.8 and 3.6 xc3x85. Useful effective separations disclosed in the patent include nitrogen from methane, O2 from argon, and O2 from N2. Each of these separations may represent the heart of a significant commercial process. Olefin/paraffin separations of hydrocarbons having the same carbon content, e.g. ethylene/ethane, are not expressly disclosed.
Separations of fluid mixtures (gases or liquids) by adsorption utilizing the ETS-type molecular sieves have been proposed in which the molecular sieve is utilized in the form of a bed, typically fixed, through which the mixture to be separated flows. Both pressure swing adsorption (PSA) and thermal swing adsorption (TSA) have been suggested to effect separation of one or more fluids from mixtures containing same. Recently, however, ETS-type molecular sieve membranes have been developed and used for molecular separation in both gas and liquid state. Copending, commonly assigned U.S. Ser. Nos. 09/663,827, now U.S. Pat. No. 6,395,067; U.S. Ser. No. 09/663,828, now U.S. Pat. No. 6,340,433; and U.S. Ser. No. 09/663,829, all filed Sep. 15, 2000, disclose ETS-type membrane preparations and uses.
Membranes formed from ETS-4 molecular sieve are particularly useful inasmuch as the pores of the ETS-4 membranes can be systematically contracted under thermal dehydration to form CTS-1-type materials as disclosed in U.S. Pat. No. 6,068,682. Under thermal dehydration, the pore size of ETS-4 can be systematically controlled from about 4 xc3x85 to 2.5 xc3x85 and sizes therebetween and frozen at the particular pore size by ending the thermal treatment and returning the molecular sieve to ambient temperature.
It is therefore a general object of the present invention to provide an improved process which overcomes the aforesaid problem of prior art methods for production of unsaturated hydrocarbons, e.g. olefins, from thermal cracking of hydrocarbon feedstocks, which olefin can be used for manufacture of polymeric materials using higher activity catalysts.
It is another object of the present invention to provide an improved process for the separation of olefins from paraffins of the same carbon number as an alternative method to the conventional low temperature, energy intensive distillations as are presently used.
It is a further object of the present invention to conduct olefin/paraffin separations to separate similar boiling hydrocarbons of the same carbon number utilizing a solid molecular sieve adsorbent which has controlled pore size.
It is another object of the present invention to provide ethylene/ethane separations utilizing unique titanium silicate solid adsorbents which have pore sizes which can be controlled from about 2.5 to 4.0 xc3x85.
It is yet another object of the present invention to provide a novel method of separating C2 hydrocarbons from each other utilizing a solid titanium silicate adsorbent having controlled pore size.
Other objects and advantages of the invention will become apparent upon reading the following detailed description and appended claims.
Economical processes are disclosed for the separation of unsaturated hydrocarbons, in particular, ethylene such as produced by thermal cracking of hydrocarbons. The processes of the present invention are for the purposes of replacing or at least augmenting prior art olefin/paraffin low temperature distillation processes which have very large operational energy costs. More specifically, the invention is directed to separating olefins from feed streams containing paraffins by passing a gaseous feed mixture containing the respective olefin and paraffin of same carbon number in contact with a titanium silicate molecular sieve, namely, ETS-4 which has been heat treated to CTS-1 of the desired pore size for the separation. The CTS-1 adsorbent can be in the form of a particulate bed wherein the olefin/paraffin separation is accomplished by pressure swing adsorption (PSA) or the CTS-1 molecular sieve can be in the form of a membrane in which the pore size selectively allows one of the components to pass through the membrane as product and the other to be retained as retentate.
In another aspect of the present invention, the separation of acetylenic components from a gas mixture of ethylene and/or propylene can be achieved by contacting the gas mixture with the titanium silicate CTS molecular sieve which has been heat treated to the desired pore size to achieve separation.